As a firing method used for manufacture of a low-temperature fired ceramic substrate, the constrained firing method (the non-contracting firing method) is known. The constrained firing method has been significantly more often used in recent years as the method is characterized in that the contraction of a substrate in the plane direction (the XY direction) can be reduced to increase dimensional accuracy and flatness. In general, the constrained firing method is known as a method of manufacturing a low-temperature fired ceramic substrate, the method comprising layering an alumina green sheet for constrained firing which is not sintered at 800 to 1000° C. as a constraining layer on at least one surface of a low-temperature fired ceramic green sheet layered product before firing, performing firing at 800 to 1000° C., and then removing the constraining layer. It may be a pressure method comprising pressurizing a ceramic substrate in the thickness direction (the Z direction) when firing, or may be a non-pressure method comprising firing without pressurization. In the pressure method, a low-temperature fired ceramic substrate having higher dimensional accuracy and flatness can be manufactured since a uniform pressure is applied to a ceramic substrate during firing.
On the outermost layer of a low-temperature fired ceramic substrate, a surface conductor such as a surface wiring pattern and a conductor for a wire bonding pad is typically formed with a conductive paste. As the conductive pastes, Ag based conductive pastes such as Ag, Ag—Pd and Ag—Pt, Cu based conductive pastes and Au based conductive pastes are commonly used which are low-melting point noble metals having a low value of electrical resistance and having excellent electrical properties.
Methods of forming a surface conductor with a conductive paste include a co-firing method comprising integrally and simultaneously firing with a low-temperature fired ceramic green sheet layered product, and a post-attachment firing method comprising printing and firing after firing a low-temperature fired ceramic green sheet layered product. The co-firing method, which has fewer steps than the post-attachment firing method due to the lack of the post-attachment step, is superior in terms of better working efficiency, lower production cost and the like. Therefore, recently, the co-firing method has been becoming the mainstream.
In a case where a surface conductor is formed by co-firing using the constrained firing method, residual materials of a constraining layer present on the surface conductor after firing are removed by grinding using a technology such as sandblasting.
Then, in order to obtain reliable electric connection by improving solderability, bonding properties, thermal resistance and the like, a plated film such as Ni, Sn and solder is formed on the surface of a surface conductor by performing plating treatment.
A large number of conductive pastes for forming a surface conductor on a ceramic substrate to be constrained-fired have been reported to date. For example, reported is a conductor powder mainly containing an Ag powder or an Ag based alloy powder, wherein 0.005 to 0.050 wt % of Rh relative to 100 wt % of the conductor powder, and the Ag based powder has a mean particle diameter of primary particles of 1.5 to 4.5 μm as measured by the electron microscopy method and a mean particle diameter of aggregated particles of 5.0 to 12 μm as measured by the centrifugal sedimentation method, and wherein the contraction behavior of a printed conductor when firing is such that the contraction percentage is 2.0 to 10.5% until the temperature increases from 400° C. to 700° C., and the contraction percentage is 10.0 to 21.1% until the temperature increases from 400° C. to 900° C. (see Patent Literature 1). The above conductive paste can reduce the warpage of a fired substrate by allowing the contraction behavior of an Ag based conductive paste to be close to that of a low-temperature fired ceramic green sheet without increasing the loading amounts of glass frit and metal oxides, and can maintain a preferred value of electric resistance of a printed conductor to improve the quality and yield.
Further, reported is a conductive paste for improving the adherence between a substrate and a surface conductor, mainly comprising an Ag powder having a median diameter of from 2.0 μm to 7.0 μm; an inorganic binder mainly comprising glass frit having a softening point of from 650° C. to 800° C.; and an organic vehicle, wherein the glass frit is a SiO2—B2O3—R2O based glass wherein R comprises at least one of Li, Na and K, and the SiO2 is contained in the glass frit in an amount of 70% or more by weight ratio, and the loading amount of the glass frit is from 1.5% to 5.0% by weight ratio relative to the Ag powder, and the ratio of the median diameter of the glass frit to that of the Ag powder is 0.4 or less (see Patent Literature 2)
Further, reported is a conductive paste for controlling the deformation and warpage of a ceramic substrate, wherein a post-degreasing contraction occurs in a period from degreasing during firing to the end of firing, and at least one contraction controlling material selected from Pt, Rh, Cu, Pd, Ni and Au is contained, provided that 1 wt % or less in a case where the contraction controlling material is Rh, 5 wt % or less in a case where the contraction controlling material is Pt, and 5 wt % or less in a case where the contraction controlling material is Cu (see Patent Literature 3).
In addition, a conductive paste for sufficiently assuring the adhesive strength of a fired surface conductor against a ceramics substrate and for reducing the occurrence of poor plating is reported, the conductive paste comprising Ag as a main component, a noble metal solid-dissolvable in Ag and a glass component (see Patent Literature 4).